Prime-boost immunization strategies against HIV-1

Abstract: Since the start of the global HIV epidemic in 1981, more than twentyfive million people have succumbed to AIDS-related diseases. The epidemic is most severe in Sub-Saharan Africa, where twentyfive million people are currently infected and more than two million died during 2006. An indication of the epidemic s severity is that every hour the virus infects between 300 and 800 people. It is clear that an effective prophylactic vaccine is desperately needed. Attempts to utilize classical vaccine strategies against HIV-1 have proven inefficient or potentially harmful and researchers have therefore been obliged to explore novel vaccine approaches. Genetic immunization is a new way of inducing immune responses against antigens deriving from microbial pathogens or tumors. The gene encoding the antigen of interest is introduced into the body by means of an expression vector, which commonly is a recombinant bacterial plasmid or an attenuated recombinant microbe. This method of immunization has the primary benefit of inducing an immune response that mimics the response to natural infection with an intracellular pathogen. The primary strategy we use to induce high levels of broadly reactive immune responses against HIV-1 is immunization with naked DNA encoding multiple viral antigens in combination with potent adjuvants. The immune responses that are induced against plasmid-encoded antigens can be significantly augmented by subsequently boosting with additional vaccine modalities, an immunization protocol referred to as heterologous prime boost. Specifically, we show in mice that it is possible to obtain both humoral and cellular immune responses to all plasmid-encoded HIV-1 antigens in a multi-plasmid vaccine. The cytokine granulocyte macrophage-colony stimulating factor acted as an adjuvant. We also show that spatial separation of the vaccine components could augment the immune responses to some of the included antigens. Further, the responses induced by the multi-plasmid vaccine were efficiently boosted using Modified Vaccinia virus Ankara (MVA) carrying similar, but not identical, HIV-1 genes. The MVA boost resulted in significantly increased levels of HIV-1 specific antibodies as well as extremely high levels of polyfunctional CD8+ T cell responses directed against all included HIV-1 antigens. We also show the capacity of DNA and MVA to induce long-lived vaccine-specific immunological memory. Importantly, DNA was shown capable of efficiently boosting an immune response primed almost one year earlier by the same DNA construct. Moreover, the capacity of different vaccine protocols to induce protection against a cell-based HIV-1 challenge was demonstrated in an experimental model. Protection against challenge was obtained by immunizing with plasmids encoding HIV-1 Gag, Env and Tat, either alone or by priming with the plasmids and subsequently boosting with the corresponding proteins. With one exception, all the vaccine constructs described here have been or are currently being evaluated in clinical trials. In our initial phase I trial, the multigene/multisubtype vaccine has shown great potential to induce HIV-1 specific cellular immune responses in humans that can be dramatically augmented and broadened by boosting with the recombinant MVA. This vaccine protocol is currently being evaluated in a phase I trial in Tanzania. Overall, the preclinical data presented in this thesis have translated well into immunogenicity in clinical trials.